This quest is not available in game.
Use the Portal to the Undercity inside Stormwind Keep to teleport to the Undercity. Report to Broll Bearmantle when you arrive at your destination.
I sent Broll and Valeera to the Undercity before you left for Orgrimmar. In fact, they're now stationed outside the sewer entrance to the ruins, awaiting orders.
Lady Proudmoore will open a portal to our forces outside the Undercity. Go there and report to Broll. Jaina and I will be close behind.